The present invention concerns the performance of loopback tests upon data transmission paths of the type designed to synchronously transmit data in a first direction at a higher, first data transmission rate but designed to asynchronously transmit data in the opposite second direction at a lower, second data transmission rate. The intended purpose of a loopback test performed upon such a data transmission path is to generate information concerning the quality of the data transmission path.
In the case of data transmission paths operating in fully duplex manner, a loopback circuit for transmission of a test waveform can be established by transmitting the test waveform from the first or testing station in the first-direction channel to the second station, and feeding the test waveform, as received at the receiver output of the second station, back into the transmitting input of the second station for transmission back to the first or testing station via the second-direction channel of the data transmission path. Evaluation of the quality of the data transmission path constituted by the first-direction and second-direction data transmission channels, such quality evaluation involving for example criteria such as bit error rate, is then performed based upon a comparison between the test waveform as reconstituted at the receiver output of the first station and the test waveform as originally applied to the transmitting input of the first station.
The performance of such loopback test begins to be problematic when the two data transmission channels are designed for respective data transmission rates which markedly differ from each other. For example, there exist data transmission paths whose first-direction channel is designed for a maximum data transmission rate of 1200 bits/sec, but whose second-direction channel is designed for a considerably lower maximum data transmission rate of 75 bits/sec. The transmission of data via the higher-rate channel can be performed synchronously in accordance with a predetermined clocking schedule or else asynchronously. With such data transmission paths, the loopback test can only be performed using the data transmission rate of the lower-rate channel. However, this leads to the disadvantage that the higher-rate channel is not being tested under realistic conditions of operation. For example, errors of transmitted data such as would develop as the data transmission rate approaches to the maximum value for which the higher-rate channel is designed, simply do not have the opportunity to develop.